Regarding dielectric filters having an outer conductor and input/output electrodes formed on an outer surface of a dielectric block and inner conductors formed inside of the dielectric block to constitute a plurality of TEM-mode (transverse electromagnetic mode) resonators, Patent Document 1 discloses a dielectric filter that reduces coupling caused by stray capacitance between input/output electrodes and increases external coupling capacitance.
An example of a configuration of a dielectric filter disclosed in Patent Document 1 will be described based on FIG. 1.
In FIG. 1, a dielectric filter 1 has an outer conductor and input/output electrodes 7 and 8 formed on an outer surface of a rectangular dielectric block 2 and inner-conductor holes 3 and 4 formed inside thereof. External coupling capacitance is determined based on a size of an area where the inner conductors formed in the inner-conductor holes and the input/output conductors face each other. Accordingly, to increase the external coupling capacitance, the two input/output electrodes 7 and 8 are formed so as to detour from a mounting surface (upper surface in FIG. 1) against a mounting board to respective lateral faces.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-162212
However, in such a dielectric filter having an outer conductor formed on an outer surface of a rectangular dielectric block, transverse electric mode (TE mode) resonance is also generated in a space formed by the dielectric block and the outer conductor formed on the outer surface thereof in addition to TEM-mode resonance, which is originally utilized. This TE mode resonance is determined based on the size and shape of the dielectric block and may exert a harmful effect on a filter characteristic depending on a condition.
FIG. 2 shows a state of an electric field and a magnetic field in a TE101 mode, which is one kind of the TE mode. In FIG. 2, a broken-line loop represents a magnetic field loop, which rotates in a plane parallel to a mounting surface of the dielectric filter 1. An electric field is vertical to the magnetic field. The magnetic field of this TE101 mode is trapped inside of the dielectric block. However, since one surface of the dielectric block is an open circuit surface 21 (FIG. 1), the magnetic loop extends to the outside from the open circuit surface.
A higher-order TE mode is also generated. For example, when a horizontal length of the dielectric block is longer than a vertical length in FIG. 2, a TE201 mode, in which two magnetic field loops lie in the horizontal direction, is generated.
Such a TE mode is, as in the originally utilized TEM mode, also excited and coupled by the input/output electrodes 7 and 8. An amount of the coupling increases as the size of the input/output electrodes 7 and 8 increases.
FIG. 3 shows examples of frequency responses (GHz) of three resonant modes, namely, the TEM mode, the TE101 mode, and the TE201 mode, and of a transmission characteristic S21(dB) between the two input/output electrodes 7 and 8. The transmission characteristic S21(dB) is a coupled result of the responses.
In general, a TE101-mode resonance frequency is higher than a TEM-mode resonance frequency. A TE201-mode resonance frequency appears at a higher position than the TE101-mode resonance frequency. Since the input/output electrodes 7 and 8 cause coupling to an electric field of the TE mode (particularly, the TE101 mode) as well as an electric field of the TEM mode, attenuation of the transmission characteristic of the dielectric filter 1 worsens in an attenuation band compared with a characteristic resulting only from the originally utilized TEM mode.
FIG. 4 shows actually measured examples of the frequency response in GHz. In FIG. 4, a broken line with “TEM” represents an estimated characteristic resulting only from the originally utilized TEM mode, whereas a curve with “TE101” represents an estimated characteristic resulting only from the TE101 mode. A curve with “F” represents a transmission characteristic (S21) in dB between the input/output electrodes 7 and 8 (FIG. 1). The transmission characteristic is hardly influenced by the TE101 mode in a pass band of the dielectric filter, shown by “A”. However, attenuation significantly worsens in a neighboring frequency band, shown by B, on a higher side of the pass band. In addition, as shown by “C”, attenuation also worsens on a lower side of the pass band by approximately 15-20 dB to be influenced the response of the TE101 mode.